Temperature Under–and Over-Range Sensing with A Window Comparator

The window can be used to monitor a TEC operating temperature and indicate an out-of-range condition. To indicate an over temperature or under-temperature, this circuit uses the separated outputs. If the output’s logic is HIGH, the condition is over-temperature, if it is LOW, the condition is the TEC temperature is within its safe operating range. Here is the circuit :

This circuit uses dual comparator with open-collector. We can use single OPA340 or dual OPA2340 CMOS op-amp which offers higher accuracy. To simulate the voltage appearing across a thermistor that measures the laser/TEC temperature, this circuit uses VIN. If a 10kO at 25°C thermistor is excited by 100µA from a REF200 current reference, it will read 1.000V across the thermistor at 25°C.  A low bias current precision CMOS op amp was used as a comparator, to prevent loading of the thermistor. This circuit is operated on +5V supply.

We can obtain  additional sensitivity by sensing an instrumentation amplifier (IA) output (this is usually the amplified error signal that is used by the control loop) instead of connecting the window comparator directly to the thermistor. [Circuit diagram source: Burr-Brown Products Application Report]